Series wound a.c. motors have both the field and armature windings excited from the line. Such motors employ brushes to supply current through the commutator and to the rotating armature windings. These motors are commonly known as universal motors and are regularly used in many commercial models of sewing machines, typewriters, drills, saws and other small appliances.
One outstanding characteristic of this type of motor is its high power and high speed per unit of weight. The speed of a universal motor is not particularly controlled or regulated by the a.c. frequency of the power source, or the number of poles in the motor structure. The principal limitation which governs the speed of the universal motor is the total resistance of the motor. In most commonly known universal motors which are sold commercially, the highest horse power output will occur in the speed range of 5,000 to 10,000 r.p.m. Under no-load conditions, various universal motors will run in the speed ranges of 18,000 to 25,000 r.p.m.; and universal motors used to operate winches will have a no-load speed of approximately 18,000 r.p.m.
Most universal motors are not subjected to an overspeed condition, but universal motors used to operate winches are subjected to an overspeed condition wherein the speed of the motor may be in the range up to approximately 25,000 r.p.m. This overspeed condition is caused when a physical load is being lowered by the winch, such that the physical load and the motor are both urging the winch spool or drum in the same direction. This may be referred to as an overhauling load condition.
The high speed of these universal motors and the typical uses of these motors may require that braking be accomplished to stop the armature whenever the motor is disconnected from the line. Dynamic braking of universal motors has been accomplished previously by proper switching when the source of power is disconnected. The switching disconnects the line from the input terminals of the motor; the connections to the commutator brushes of the armature are reversed so as to instantaneously cause the motor to commence acting as a generator; and a low resistance is connected across the input terminals of the motor so as to act as a load for the series connected field and armature windings which are now operating as a generator. The armature will very rapidly decelerate, and in most situations will completely stop.
Such universal motors have been commercially used to power small compact winches. A small motor turning at high speed can develop an adequate amount of mechanical power output when applied to the winch drum or spool through a pair of tandemly arranged worm and gear speed reductions, so that the winch and motor combination may continue to have a reasonable size and weight in relation to its power output and general usefulness.